The uropygial gland, informally known as the preen gland or the oil gland, is a bilobed sebaceous gland possessed by the majority of birds used to distribute the gland's oil through the plumage by means of preening. It is located dorsally at the base of the tail (between the fourth caudal vertebra and the pygostyle) and is greatly variable in both shape and size. In some species, the opening of the gland has a small tuft of feathers to provide a wick for the preen oil (see below). It is a holocrine gland enclosed in a connective tissue capsule made up of glandular acini that deposit their oil secretion into a common collector tube ending in a variable number of pores (openings), most typically two. Each lobe has a central cavity that collects the secretion from tubules arranged radially around the cavity. The gland secretion is conveyed to the surface via ducts that, in most species, open at the top of a papilla (nipple-like structure).
Etymology From uropygium: Medieval Latin, from Ancient Greek οὐροπύγιον (ouropugion), from οὐρά (oura) 'tail' and πυγή (puge) 'rump'.
Distribution amongst species The gland is invariably present during embryonic development, whereas it can be vestigial in adults of certain orders, families, genera and species. Some or all species in at least nine families of birds lack a uropygial gland, mostly the ones unable to fly or the ones that produce powder down for feather maintenance. These include kiwis (Apterygidae), emu (Dromaiidae), ostriches (Struthionidae), rheas (Rheidae), cassowaries (Casuariidae), mesites (Mesitornithidae), bustards (Otididae), pigeons and doves (Columbidae), amazon parrots (Psittacidae), frogmouths (Podargidae), and woodpeckers (Picidae). These birds typically find other means to stay clean and dry, such as taking a dust bath. Researchers have been unable to correlate the presence or absence of the uropygial gland with factors such as distribution, climate, ecology, or flightlessness.
Gland secretion
The uropygial gland secretes an oil (preen oil) through the dorsal surface of the skin via a grease nipple-like nub or papilla. The oil contains a complex and variable mixture of substances formed greatly of aliphatic monoester waxes, formed of fatty acids and monohydroxy wax alcohols. However, some types of diester waxes called uropygiols and containing hydroxy fatty acids and/or alkanediols exist in the secretions of the uropygial gland of some groups of birds. Preen gland secretion of some, but not all, birds have shown to be antimicrobial. Some birds harbor bacteria in their preen gland, which to date, have (exclusively) been isolated from preen glands (e.g. Enterococcus phoeniculicola and Corynebacterium uropygiale). Some of those bacteria add to the antimicrobial properties of preen wax. A bird will typically transfer preen oil to its body during preening by rubbing its beak and head against the gland opening and then rubbing the accumulated oil on the feathers of the body and wings, and on the skin of the feet and legs. Tailward areas are usually preened utilizing the beak, although some species, such as budgerigars, use the feet to apply the oil to feathers around the vent.
History Emperor Frederick II, in his thirteenth-century treatise on falconry, was seemingly the first to discuss the function of the uropygial gland of birds. He believed that its product not only oiled the plumage but also provided a poison which was introduced by the claws of hawks and owls thus bringing quicker death to their prey. However, studies in 1678 on the question of the toxic nature of the uropygial gland secretion found no evidence to support Frederick's contention.
Function Several researchers have reported differences in the relative gland weights attributing them to factors like seasonal changes, habitat, body weight, inter-individual variations, and sex. Significant differences are found in the relative gland size between males and females in most species, however, no coherent explanation has as yet been found for these results. Many ornithologists believe the function of the uropygial gland differs among various species of birds.
Feather and body integrity Preen oil is believed to help maintain the integrity of feathers. In waterbirds, preen oil maintains the flexibility of feathers and keeps feather barbules from breaking. The interlocking barbules, when in good condition, form a barrier that helps repel water (see below). In some species, preen oil is also believed to maintain the integrity of the horny beak and the scaly skin of the legs and feet. It has also been speculated that in some species, preen oil contains a precursor of vitamin D; this precursor is converted to vitamin D by the action of sunlight and then absorbed through the skin.
Courtship and pheromone production Some researchers have postulated that the change in preen oil viscosity may be related to the formation of the more brilliant plumage required for courtship, although later research did not find support for this idea. The results of other studies suggest that the gland in females may be involved in the production and secretion of lipids with female pheromone activity.
Waterproofing effect The uropygial gland is strongly developed in many waterbirds, such as ducks, petrels, pelicans and in the osprey, and sometimes in non-aquatic species, such as the oilbird. A study examining the gland's mass relative to body weight in 126 bird species showed the absence of a clear-cut correlation between the gland's mass and the degree of birds' contact with water. Anecdotal reports indicating that the waterproofing effect of the hydrophobic uropygiols might be increased by electrostatic charge to the oiled feather through the mechanical action of preening are not supported by scientific studies.
Antiparasitic effect
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